TM 5-3825-226-24OPERATIONGENERALThe compressor runs continuously while the engine isrunning but actual compression of air is controlled by agovernor which stops or starts the compression of air byloading or unloading the compressor in conjunction withits unloading mechanism. This is done when the airpressure in the system reaches the desired maximum orminimum pressures.FIGURE 4INTAKE AND COMPRESSION (Loaded)During the down stroke of the piston, a slight vacuumcreated above the piston causes the inlet valve to moveoff its seat. Atmospheric air is drawn in through thecompressor intake, by the open inlet valve, and on topof the piston (Fig. 4). As the piston starts its upwardstroke, the air that was drawn in on the down stroke isbeing compressed.Now, air pressure on top of the inlet valve plus the forceof its spring, returns the inlet valve to its seat. Thepiston continues the upward stroke and compresses theair sufficiently to overcome the discharge valve springand unseat the discharge valve. The compressed airthen flows by the open discharge valve, into thedischarge line and on to the reservoirs (Fig. 5).As the piston reaches the top of its stroke and startsdown, the discharge valve spring returns the dischargevalve to its seat. This prevents the compressed air inthe discharge line from returning to the cylinder bore asthe intake and compression cycle is repeated.NON-COMPRESSION (Unloaded)When the air pressure in the reservoir reaches the highpressure setting of the governor, the governor opens,allowing air. to pass from the reservoir through thegovernor and into the cavity beneath the unloaderpistons. This lifts the unloader pistons and plungers.The plungers move up and hold the inlet valves off theirseats (Fig. 6).With the inlet valves held off their seats by the unloaderpistons and plungers, air is merely pumped back andforth between the two cylinders. When air is used fromthe reservoir and the pressure drops to low pressuresetting of the governor, the governor closes and in doingso exhausts the air from beneath the unloader pistons.The unloader saddle spring forces the saddle, pistonsand plungers down and the inlet valves return to theirseats. Compression is then resumed.2-22